Fission yields - International Centre for Theoretical...

Fission yields

Stephan Pomp [email protected]

Department of physics and astronomy Uppsala University

S. Pomp, ICTP Trieste, Oct 2017

Outline

• Some literature sources

• Introduction: – Phenomenology, time scale, obserables, challenges, data needs, …

• Fission yields (FY): – Definitions, trends, ….

• Overview on model codes

• Overview on experimental methods

• Examples of experimental efforts – 2E with FGIC, 2E-2v (VERDI and FALSTAFF), IGISOL and more …


Literature: some suggestions

http://urn.kb.se/resolve?urn=urn%3Anbn%3Ase%3Auu%3Adiva-328484

Fission in general: • N. Bohr and J.A. Wheeler, The mechanism of nuclear fission, Phys. Rev. 56 (1939) 426. • R. Vandenbosch and J. Huizenga, Nuclear fission, Academic Press, 1973. • C. Wagemans, The nuclear fission process, CRC Press, 1991. • A.N. Andreyev et al., Nuclear fission: a review of Experimental Advances and

Phenomenology, Rep. Prog. Phys, 2017 (in press). On measurement techniques etc.: e.g. PhD theses available for free on the web: • Ali Al-Adili, PhD thesis, Uppsala, 2013. • Andrea Mattera, PhD thesis, Uppsala 2017. • Kaj Jansson, PhD thesis, Uppsala 2017. … and of course many other papers and theses from many other universities and labs


http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-328484

Outline








Fission: time line and observables

Cross section, fission yields (at various stages), prompt neutron multiplicity and energy spectra prompt gamma multiplicity and energy spectra


Energy release and time scale Kinetic energy of fragments: typically 85 % of total energy release (but large variations!) Kinetic energy to prompt neutrons: about 5 MeV (then we have fission products) Energy to prompt gammas: about 7 MeV Energy to beta decay etc: about 10 % (cumulative..) Note: neutrinos take away about 8 MeV

Figure from C. Wagemans (ed.), The Nuclear Fission Process (Boca Raton, 1991)

See S. Prussin, Nuclear Physics for Applications, p. 401 for a detailed list


The complete nuclear physics lab (aka fission)

Fission is a slow, dynamic process which can be viewed as nuclear shape evolution.

Nuclear shape evolution with Langevin equations; courtesy S. Chiba

A.V. Karpov et al., J. Phys. G: Nucl. Part. Phys. 35 (2008) 035104

Fission comprises many aspects of nuclear physics. E.g. structure of very deformed nuclei far from stability


A 3-D view of a potential landscape

From: A.V. Karpov et al., J. Phys. G: Nucl. Part. Phys. 35 (2008) 035104


Computer power and random walk

P. Möller et al., Nature 409 (2001) 785

Potential energy valleys and nuclear shapes for 234U

Five-dimensional shape representation for potential energy calculation:

These calculations can be used to calculate fission yields. J. Randrup and P. Möller, Phys. Rev. Lett. 106 (2011) 132503.


Other observables: e.g. angular distributions …

PhD thesis A. Al-Adili, Uppsala 2013

Strong fluctuations in the angular distributions in 234U(n,f) close to vibrational resonance. Resonances connected to the so-called transition states at the saddle. Also changes in mass and energy distributions where observed. May be connected to structure in potential energy landscape (fission barrier heights).


Excitation energy

237Np(n,f)

Some challenges

GEF code

New theoretical ideas: Energy sorting mechanism based on the constant temperature model Schmidt and Jurado, PRC 83 (2011) 061601(R)

Questions: Where does the excitation energy go? When does the freeze-out of fragment properties occur? Do scission neutrons exist?

Linked to that: Prompt neutrons? Isomeric yields? …

K.-H. Schmidt et al., Nucl. Data Sheets 131 (2016) 107-221

252Cf(SF)

A. Göök et al., PRC 90 (2014) 064611


T1/2 = 55.9 sec

Needs Needs: e.g., knowledge of fission yields for various fuel cycle or nucleosynthesis scenarios . Examples for reactor applications: - Other fuels and high burn-up … - Precursors for delayed-neutron emission - Long-lived fission products (final storage) - Neutron poisons (neutron absorbers)

Figure from Bertulani, Nuclear physics in a nutshell.


Needs II Needs can only be fulfilled by direct measurements combined with theoretical developments that need guidance from other experimental input (e.g. ν(A)).

A. Al-Adili, et al, Phys. Rev. C86 (2012) 054601

80 90 100 110 120 130 140 150 1600.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Heavy (HE) En=5 MeV

Average (AV) En=5 MeV

En=0 MeV

ν

(A)

Mass (u)80 90 100 110 120 130 140 150 160

0.6

0.8

1.0

1.2

1.4

1.6

Yiel

d ra

tio

Post neutron emission Mass (amu)

Yield ratio AV/HE 5 MeV

Impact on ν(A) on fission yields in the 234U case:

P. D

imitr

iou

et a

l, IA

EA, I

ND

C(N

DS)

-071

3 (2

016)


Outline








Yields: Definitions I

• Fragment yields: Yields prior to prompt neutron emission.

• Product yields: Yields posterior to prompt neutron emission.

neutron emission

gamma emission

scission point


Yields: Definitions II

• Independent fission yields: number of atoms of a specific nuclide produced directly in the fission process (i.e., not via radioactive decay of precursors).

Source: IAEA-TECDOC-1168 (2000) S. Pomp, ICTP Trieste, Oct 2017

Yields: Definitions III

• Cumulative fission yields: total number of atoms of a specific nuclide produced (directly and via decay of precursors).


Yields: Definitions III

• Total chain yields: defined as the sum of cumulative yield(s) of the last (stable or long-lived) chain member(s).

• Mass number yields: defined as the sum of all independent yields of a particular mass chain and are in this way distinguished from chain yields.

• Some of the most modern methods to measure fission yields provide sets of truly independent yields which - at summation - will produce mass number yields rather than chain yields.


• Light mass peak shifts as compound mass increases

• Heavy mass peak remains centered around A≈140.

• Effect of closed shells (132=50+82)

Trends I



Fig 7 in P. Möller et al., N

ature 409 (2001) 785

The model by P. Möller et al. manages to describe the experimental fact that the average mass of the heavy fragment remains roughly stable around A=140.

Open circles: calculation Filled circles: exp. data

Trends II

Lighter compound system and fewer prompt neutrons better production rates for exotic, neutron-rich nuclei

78Ni


Trends III

PhD thesis V. Sim

utkin, Uppsala 2010

Yields generally asymmetric But towards higher energies the symmetric component Increases (and gets wider). I.e. shell effects less important at high excitation energies.

higher energy of incident neutron

238U

232Th


Trends IV

Figure from K.H. Schmidt et al., Nucl. Data Sheets 131 (2016) 107


Outline








Models I Multi-Modal Random-Neck Rupture Model (Brosa model) (1) multi-channel fission (“fission modes”); different paths on potential energy

landscape, defines shape of fissioning nucleus; plus (2) random break-up of the neck.

U. Brosa et al., “Nuclear scission”, Phys. Rep 197 (1980) 167. A. Al-Adili, PhD thesis, Uppsala 2013

234U(n,f) at ~ 2 M

eV


From: Brosa et al., Phys. Rep. 197 (1990) 167 From: Ali Al-Adili, PhD thesis, Uppsala, 2013 S. Pomp, ICTP Trieste, Oct 2017

Models II GEF (General Fission) model (K.H. Schmidt et al., “General Description of Fission Observables: GEF Model Code”, Nucl. Data Sheets 131 (2016) 107)

“The GEF code aims to provide a complete description including the entrance channel

and the de-excitation of the fragments”

http://www.khs-erzhausen.de/GEF.html


http://www.khs-erzhausen.de/GEF.html

Models III

Versions of the Brosa and GEF models are part of TALYS 1.8 Other models often focus on de-excitation (prompt n and γ), e.g.: • FREYA by R. Vogt et al., https://nuclear.llnl.gov/simulation/main.html YouTube video: https://www.youtube.com/watch?v=x__MNOqdLpI • FIFRELIN by O. Litaize et al., Eur. Phys. J. A 51 (2015) 1. • CGMF by P. Talou et al., Phys. Rev. C 83 (2011) • Point-by-point model (PbP) by A. Tudora et al., e.g. Ann. Nucl.

Energy 33 (2006) 1030.


https://nuclear.llnl.gov/simulation/main.html

https://nuclear.llnl.gov/simulation/main.html

https://www.youtube.com/watch?v=x__MNOqdLpI



Review of (at the time) available experimental FY data and development of an empirical model for 233U(nth,f), 235U(nth,f), 239Pu(nth,f), 252Cf(SF) : Later extension, e.g. A.C.Wahl, LA-13928 (2002) include, e.g., nubar(A).

Wahl systematics I

A.C. Wahl, At Data and Nucl Data Tables, 39 (1988) 1-156

Yields normalized to total yield of 200%

.


Gaussian shape σA ≈ 1.5-2 σZ ≈ 0.5

Emp. model for Zp and/or A´p

A.C. Wahl, At Data and Nucl Data Tables, 39 (1988) 1-156

Corrections for even-odd effects

Wahl systematics II


Outline








Experimental methods I Stopped fragments: • Radiochemical separation

– earliest method; only long-lived products; activity measured from β-decay or γ-decay (with low resolution)

• Mass spectrometry – long-lived products only; sample evaporated and mass analyzed;

precise technique but difficult to normalize • γ-ray spectroscopy

– isotope specific; also short-lived nuclides accessible; can be used with activation technique or fast on-line mass separation

• ISOL method – can be combined with Penning trap or γ-ray spectroscopy; can

be difficult to normalize S. Pomp, ICTP Trieste, Oct 2017

Experimental methods II

Unstopped fragments: • LOHENGRIN@ILL

– reactor based – mass, charge, and

energy are measured – only one fragment – combined with, e.g.,

γ-spectroscopy

Image from

https://ww

w.ill.eu


https://www.ill.eu/

https://www.ill.eu/

Experimental methods III

Unstopped fragments: • Inverse kinematics

– accelerator based (charged particle induced fission or Coulomb excitation)

– can access fission of exotic, very short-lived nuclei

– e.g. SOFIA@GSI (both FF) and VAMOS@GANIL (one FF)

– high mass resolution (isotopic yields)

Figure: E. Pellereau et al., EPJ Web of Conf. 62, 06005 (2013)


Latest results from GSI


E. Pellereau et al. Phys. Rev. C 2017

Charges of correlated fission products from 238U

Experimental methods IV

Unstopped fragments: Experiments with (high-energy) neutron beams • More “small scale”, flexible experiments

– 2E method and 2v method – 2E-2v method

• Need for suitable targets (sizeable amounts)


2E-method

Ionization chamber GAS

charge

Anode

Measurement: e.g. with Frisch-Grid Ionization Chamber: (see, e.g., PhD thesis Ali Al-Adili, Uppsala 2013)

Inside of a FGIC:

Target (Cathode, backing)

Frisch-Grids and Anodes


Typical mass resolution: 4-6 u

2E-method • If the kinetic energies 𝐸𝐸1

𝑝𝑝𝑝𝑝𝑝𝑝 and 𝐸𝐸2𝑝𝑝𝑝𝑝𝑝𝑝 of the two fission fragments

are measured, the fragment masses can be obtained from momentum conservation (in the c.m. system).

𝒎𝒎𝟏𝟏,𝟐𝟐𝒑𝒑𝒑𝒑𝒑𝒑 = 𝒎𝒎𝑪𝑪𝑪𝑪 ∙

𝑬𝑬𝟐𝟐,𝟏𝟏𝒑𝒑𝒑𝒑𝒑𝒑

𝑬𝑬𝟏𝟏𝒑𝒑𝒑𝒑𝒑𝒑 + 𝑬𝑬𝟐𝟐

𝒑𝒑𝒑𝒑𝒑𝒑

Problem: Measured: 𝐸𝐸𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 −→ need to make an assumption on the energy change due to neutron emission.


2E-method

𝑬𝑬𝟏𝟏,𝟐𝟐𝒑𝒑𝒑𝒑𝒑𝒑 𝒊𝒊 + 𝟏𝟏 = 𝑬𝑬𝟏𝟏,𝟐𝟐

𝒑𝒑𝒐𝒐𝒐𝒐𝒐𝒐 ∙𝒎𝒎𝟏𝟏,𝟐𝟐

𝒑𝒑𝒑𝒑𝒑𝒑 𝒊𝒊

𝒎𝒎𝟏𝟏,𝟐𝟐𝒑𝒑𝒐𝒐𝒐𝒐𝒐𝒐 𝒊𝒊 + 𝟏𝟏

𝒎𝒎𝟏𝟏,𝟐𝟐𝒑𝒑𝒐𝒐𝒐𝒐𝒐𝒐 𝒊𝒊 + 𝟏𝟏 = 𝒎𝒎𝟏𝟏,𝟐𝟐

𝒑𝒑𝒑𝒑𝒑𝒑 𝒊𝒊 − ν𝟏𝟏,𝟐𝟐 𝒎𝒎𝟏𝟏,𝟐𝟐𝒑𝒑𝒑𝒑𝒑𝒑

Use iterative procedure assuming kinetic energy scales with mass of fragment/product to get better value for the fragments energy:

𝒎𝒎𝟏𝟏,𝟐𝟐𝒑𝒑𝒑𝒑𝒑𝒑 𝒊𝒊 + 𝟏𝟏 = 𝒎𝒎𝑪𝑪𝑪𝑪 ∙

𝑬𝑬𝟐𝟐,𝟏𝟏𝒑𝒑𝒑𝒑𝒑𝒑 𝒊𝒊 + 𝟏𝟏

𝑬𝑬𝟏𝟏𝒑𝒑𝒑𝒑𝒑𝒑 𝒊𝒊 + 𝟏𝟏 + 𝑬𝑬𝟐𝟐

𝒑𝒑𝒑𝒑𝒑𝒑 𝒊𝒊 + 𝟏𝟏

Do until changes are small


neutron multiplicity as function of fragment mass (input from some model)

2v-method • Similar to the previous method but instead of the energies, velocities

are measured. • Assuming we have the fragment (pre neutron-emission) velocities,

momentum conservation gives:


v𝟐𝟐,𝟏𝟏𝒑𝒑𝒑𝒑𝒑𝒑

v𝟏𝟏𝒑𝒑𝒑𝒑𝒑𝒑 + v𝟐𝟐


And same problem again: Measured: v𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 −→ need to assume that velocity is unchanged by neutron emission and use an iterative procedure …

How to? Use, e.g. PPACs (Parallel Plate Avalanche Chambers) as start and stop detectors.


Measurement of energy and velocity of both products. Product masses are readily obtained: Like in 2v-method: assuming that, on average, velocities are unchanged by neutron emission, pre neutron-emission masses are obtained (momentum conservation):

2E-2v-method I

𝒎𝒎𝟏𝟏,𝟐𝟐𝒑𝒑𝒐𝒐𝒐𝒐𝒐𝒐 =

𝟐𝟐 ∙ 𝑬𝑬𝟏𝟏,𝟐𝟐𝒑𝒑𝒐𝒐𝒐𝒐𝒐𝒐

v𝟏𝟏,𝟐𝟐𝒑𝒑𝒐𝒐𝒐𝒐𝒐𝒐 𝟐𝟐


v𝟐𝟐,𝟏𝟏𝒑𝒑𝒑𝒑𝒑𝒑

v𝟏𝟏𝒑𝒑𝒑𝒑𝒑𝒑 + v𝟐𝟐



Now comes the thing: ν(𝒎𝒎𝒑𝒑𝒑𝒑𝒑𝒑) can be obtained: (event-by-event) and also: This works rather well but …

2E-2v-method II

ν𝟏𝟏,𝟐𝟐 = 𝒎𝒎𝟏𝟏,𝟐𝟐

𝒑𝒑𝒑𝒑𝒑𝒑 −𝒎𝒎𝟏𝟏,𝟐𝟐𝒑𝒑𝒐𝒐𝒐𝒐𝒐𝒐

𝒎𝒎n

𝑬𝑬𝟏𝟏,𝟐𝟐𝒑𝒑𝒑𝒑𝒑𝒑 =

𝟏𝟏𝟐𝟐

𝒎𝒎𝟏𝟏,𝟐𝟐𝒑𝒑𝒑𝒑𝒑𝒑 v𝟏𝟏,𝟐𝟐

𝒑𝒑𝒐𝒐𝒐𝒐𝒐𝒐 𝟐𝟐


However: the “unchanged velocity” assumption leads to a width of the mpre uncertainty of about 0.8 u and a bias …. Good news: this can be correct for.

2E-2v-method III

K. Jansson et al., arXiv:1709.07443

252Cf

Simulation with “perfect” experimental resolution (pseudo-data from GEF code)


2E-2v-method IV

K. J

anss

on e

t al.,

arX

iv:1

709.

0744

3

Conclusion: The 2E-2v method is well-suited to measure mpost and mpre with good resolution (about 1u). Hence: very good way to obtain neutron multiplicity as function of fragment mass.


Outline








Outline

• Some literature sources • Introduction:

– Phenomenology, time scale, obserables, challenges, data needs, …


• Overview on model codes • Overview on experimental methods • Examples of experimental efforts

– 2E with FGIC, 2E-2v (VERDI and FALSTAFF), IGISOL


n

FF2

6

6

+ 1.0 kV

0 V

+ 1.0 kV

0 V

- 1.5 kV

CATHODE

30

_X

FF1

n

NEUTRON BEAM

ANODE 1

ANODE 2

θ234U

Backing

30

ANODE 1

ANODE 2

GRID 2

GRID 1

[mm]

E

θ

θ

E

FY using the 2-E method

FGIC (Frisch-Grid Ionization Chamber) ◎ ~4π geometrical efficiency.

◎ Very simple operation/analysis

based on conservation of momentum and mass.

◎ Poor mass resolution (4-6 u).

◎ Assumption on neutron emission needed.


Selected results ◎ Angular distributions ◎ Mass yields

◎ TKE distributions

◎ Fission mode

parameteri-zations

A. Al-Adili et al.,

Phys. Rev. C 93, 034603, (2016)


FY using the 2-E method

2E-2v with VERDI (VElocity foR Direct particle Identification)

Silicon detector (STOP) MCP (START) TOF


Energy 2 arrays of 16 Si detectors,

Velocity (Time-of-Flight)

Start: Electrons emitted from

target detected by Micro

Channel Plate (MCP)

Stop: Si detector


2E-2v with VERDI ◎ Good mass resolution (1-2 u)

◎ No assumption on neutron emission

needed. -> ν(A) as bonus!

◎ Rather complicated/folded analysis.

◎ <1% geometrical efficiency.

FALSTAFF (Four arm cLover for the Study of Actinide Fission Fragments)


o Large solid angle (~1% of 4π) o Good position resolution (1.2 mm)

Mass before evaporation 2V method TOF : Good time resolution (σ) <150 ps Mass after evaporation EV method

Energy & TOF o Good energy resolution (~1%) o Charge identification (dE profile)

at NFS in 2018

Slide courtesy Diane Doré Spectrometer for fission fragment detection in coincidence

o Kinetic energy o Masses BEFORE and AFTER evaporation (-> ν(A)) o Charge

Similar efforts • SPIDER (LANL); Meierbachtol et al., NIM A 788 (2015) 59

• STEFF (Univ. Manchester); http://t2.lanl.gov/fiesta2014/presentations/Smith.pdf


http://t2.lanl.gov/fiesta2014/presentations/Smith.pdf

COSI VAN TUTTE A. Oed et al. NIM 219 (1984) 569


Figures courtesy P. Schillebeeckx (PhD thesis)

E-v method, i.e. one fragment measured post-masses only (nth,f); time resolution : 100 ps; masses resolved: ∆m/m = 0.6%

• Basic idea and motivations • IGISOL and JYFLTRAP • Proton data • The neutron source • First experimental results • Extraction of FF angular momenta • Summary and outlook


IGISOL

FY and IYR at IGISOL

• Combine Isotope Separation OnLine technique with mass-resolving power of a Penning trap to obtain FY and even IYR.

• Data on isotopic yields and independent yields are scarce (and hard to obtain)

• Isomeric yields ratios (IYR) can provide knowledge on spin distributions in various fissioning systems and can probe, e.g., dependence on fission modes, fissioning system, excitation energy.


Further motivation and goals

• IYR are relevant for anti-neutrino spectra from reactors … [A. A. Sonzogni, et al. Phys. Rev. Lett. 116:132502 (2016)]

• … and are relevant in the r-process (in cases where the isomer is not in thermal equilibrium with the ground state)

And on the “fission side”:

• Can we gain insight in the energy dependence on IYR? • Possibility/goal to study various entrance channels:

– > (p,f), (n,f) and (SF)


Why Penning traps

• Possibility to measure isotopic yields and isomeric ratios for various systems, entrance channels, excitation energies

• The method is fast (order of 100 ms) and based on direct ion counting (no need for γ-decay schemes)

• Routinely a mass resolution <1 MeV is achieved

• The Phase-Imaging Ion-Cyclotron-Resonance technique will allow for resolving mass difference of about 50 keV and a precision in mass measurements on the 10 keV scale

[S. Eliseev et al., Phys. Rev. Lett. 110, 082501 (2013)]

• Thus a large range of isomers, covering, e.g., various fission modes, may be studied


Examples of possibilities …


Below is part of a tentative list of isomers that can with PI-ICR technique at JYFLTRAP. Technique was recently successfully tested.

JYFL Accelerator News, Vol. 25, No 2. (2017) http://users.jyu.fi/~pheikkin/Newsletter/Newsletter.pdf

……

http://users.jyu.fi/%7Epheikkin/Newsletter/Newsletter.pdf

http://users.jyu.fi/%7Epheikkin/Newsletter/Newsletter.pdf

IGISOL and JYFLTRAP @JYFL MCC30/15 Cyclotron p: 18 – 30 MeV @ 100 μA D: 9 – 15 MeV @ 50 μA

Also possible to use K-130 cyclotron, Could give ~ 4000 hrs/yr

IGISOL = Ion Guide Isotope Separation OnLine JYFLTRAP = JYväskylä physics Laboratory penning TRAP

Fission chamber and ion guide


-10 kV

Beam

Fissile target

200 mbar He as buffer gas


More on the fission chamber


• Chemistry can matter – probability of 1+ charge state?

1+/2+

132Sn 1.19 ± 0.17

132Sb 2.78 ± 0.42

Simulations …


Simulations of mass- and energy dependence of fission product extraction efficiencies. As input to the simulations realistic fission data were used (GEF code). Dependence of stopping efficiency on mass and energy is found to be small compared to other systematic uncertainties.

A. Al-Adili et al., EPJ A (2015) 51:59, K. Jansson et al., submitted to EPJ A

target Ni foil

First mass separation


Dipole magnet Mass resolving power M/ΔM=500

γ spectroscopy station (missing in figure) Ge detector in coincidence with β counter

RFQ-cooler and buncher


Ions are collected and then released towards the trap

H. Penttilä et al., Eur. Phys. J. A 44, 147–168 (2010)

Two stage Penning trap


Mass resolv. power M/ΔM > 105

Resolve isomers 0.5 MeV apart (and better …)

Cyclotron frequency Each nuclide/isomer is Identified by its unique frequency in the Penning trap

Detector chamber Multi-channel plate (MCP) counts all ions at given cyclotron frequency Total time from production to detection ~300 ms

How it then can look …


1.5 MeV

Coun

ts

H. P

enttilä et al., Eur. P

hys. J. A 44, 147–168 (2010)


A number of measurements for p+natU and p+232Th have been performed

H. Penttilä et al., Eur. Phys. J. A (2016) 52: 104

Done: IFY from (p,f) at 25 and 50 MeV

Example Rhodium: measured with two different reference points


IFY for natU(p,f) at 25 MeV

H. Penttilä et al., Eur. Phys. J. A (2016) 52: 104

Isotopic yield distributions adjusted to UKFY4.1

IYR in (p,f)


Performed: • First direct measurement of IYR by direct ion counting.in (p,f) • Several isomeric pairs in 238U(p,f) and Th(p,f) measured for the

first time.

V. Rakopoulos et al., ND2016 proceedings V. Rakopoulos et al., manuscript with new data from 2016 in preparation

Six isomers studied


Example: the toughest case, 81Ge

Yield(96mY) Yield(96Y)+Yield(96mY)

Isomeric yield ratios

Question: Is there a dependence on the initial system or the fission mode for some isomeric yield ratios?

V. Rakopoulos, preliminary results

Ratio =

1.5 MeV

Coun

ts

For (p,f) at 25 MeV


Now let’s go to (n,f)


A. Solders et al., NDS 119 (2014) 338. A. Mattera et al., EPJ A 53 (2017) 173.

PPAC prototype Characterization of Be(p,xn) neutron field with activation plates.

Use Be(p,xn). Thick target. “White” neutron spectrum

First we need a neutron source

Challenge: efficiency and normalization (need for simulations and measurements)


Up to six normal targets (one in each hexagon side holder), or one big (5x2 cm) foil pressed against the chamber wall.

Extensive simulations of stopping efficiency: - Neutron field from MCNPX - Ions sampled from GEF - Transport with Geant4 The “only” thing missing: charge state distributions. Measurements with implantation foils currently under analysis. Use of Cf-252 source planned.


Energy distribution and multi-chance

natU target: average energy (weighed with XS): ~ 12.4±8.8 MeV (97 ± 1) % of fissions from 238U Multi-chance fission open

(n,f) (n,nf)

(based on GEF)

….

In the (n,f) case we could not yet go …


… all the way to the trap in this run (too low proton current). Results presented here are based on γ-spectroscopy (with β coincidence) after the analyzing magnet. Six magnet settings: masses 128 – 133 studied. Data were collected during 2h each.

Sn isotopes - CFY


Relative cumulative yields.

Sn

Sb isotopes - IFY


Relative independent yields.

Sb

Only the isomer

Measured IYR


Sn-m Sn Sb-m Sb

A = 128 IT: 100 %

A = 129

A = 130

A = 131

A = 132

A = 133

There are 5 nuclides where we see both isom & GS → we can extract the IYR

Results (also still preliminary)


Sn-m Sn Sb-m Sb

A = 128 IT: 100 %

A = 129

A = 130

A = 131

A = 132

A = 133

There are 5 nuclides for which we can extract both isom & GS → we can extract the IYR

Deduce FF spin using TALYS for de-exciation


The trend of the curve depends on the spin difference between the ground state and the metastable state. For the 81Ge case Jg.s.=9/2+ & Jm=1/2+, while for the 130Sn case Jg.s.=0+ & Jm=7-.

See V. Rakopoulos et al., ND2016 proceedings

Based on an assumption of emitted neutrons, TALYS is used to predict the IYR for different initial FF spins. The obtained dependence can be compared with the experimental value.

Ackowledgements

Thanks to the nuclear reactions team in Uppsala, in particular Ali Al-Adili, Kaj Jansson, Andrea Mattera, and Vasileios Rakopoulos for providing material for this lecture.


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